In power electronics, it is sometimes necessary to measure the current through a field-effect transistor (FET). Typically this is accomplished by placing a sense resistor in electrical series with the FET and measuring the voltage drop across the resistor. For example, in new electric brake designs, the brake caliper is controlled by a screw and gear assembly connected to the motor shaft. The motor is controlled by an inverter employing a motor bridge circuit typically comprising switches in the form of FETs. For brake applications, it is necessary for the electronics to measure both motoring and regenerative current during operation of the motor. This is accomplished by measuring the voltage drop in a series shunt resistor due to these positive and negative motor currents. In this application, and in others measuring the current through a FET, such resistors are oftentimes costly and/or physically large.
It has been suggested to measure the drain-to-source voltage of the FET, which is present during the flow of current through the FET due to the presence of the on-resistance of the FET. However, the on-resistance of a FET varies significantly with temperature. In a typical FET, for example, the on-resistance at 175° C. is twice as high as it is at 25° C. This fact discourages the use of the drain-to-source voltage to sense current through a FET.